1. Technical Field
The invention relates to opiate receptors. Specifically, the invention relates to mu3 and mu4 opiate receptors as well as mu3 and mu4 opiate receptor activation and inhibition.
2. Background Information
Three general classes of cell surface opioid receptors (kappa, delta, and mu) have been described based on ligand specificity. Opioid receptors exhibiting high binding specificity for morphine have been designated mu opioid receptors. Detailed analysis of mu opioid receptors from various tissues has revealed the existence of multiple mu opioid receptor subtypes. In fact, the cDNA encoding the mu1 opioid receptor subtype has been identified. Oligonucleotides complementary to some, but not all, exons of the mu1 opioid receptor can block the effects mediated by the mu1 and mu2 receptor subtypes. Thus, the mu1 and mu2 opioid receptor subtypes appear to share exon sequences, as would be expected of splice variants. Supporting the idea of alternative splicing is the finding of a single mu gene in human and mouse chromosomal DNA. In addition, a novel rat brain mu opioid receptor subtype, designated rMOR1B, has been identified. This receptor is identical to the rat mu1 opioid receptor at its N-terminus but differs in its length and sequence at the C-terminus. Further, affinity studies demonstrated that the substrate specificity of rMOR1B is similar to that of the rat mu1 opioid receptor, but rMOR1B is more resistant to agonist-induced desensitization and has a different expression pattern in brain. The presence of another opiate receptor, designated mu3 opiate receptor, has been demonstrated pharmacologically. This mu3 opiate receptor is opioid peptide insensitive and opiate alkaloid selective. In addition, detailed binding analysis indicates that the mu3 opiate receptor is expressed by immune tissues (e.g., human monocytes and granulocytes).